A great swami once said that, "Everyone is Hindu -- only some of us know it." This is certainly an extreme, all-encompassing definition, but many great swamis will often accept people of other faiths, like Jains and Buddhists, as
Hindus. However, some Jains and Buddhists are truly atheistic (without any deity), which initially seems to be incompatible with Hinduism. Gandhi once wrote that, "If I were asked to define the Hindu creed, I should simply say:
search after Truth through non-violent means. A man may not believe in God and still call himself a Hindu." In this statement, Gandhi alludes to the more universal Hindu way of life, which may or may not include a deity. Some
would assert that any who follow a righteous path to approach Truth is Hindu, whether they realize it or not, or whether they accept it or not. There are many paths to Reality described by Hinduism, including devotion, action, and
knowledge. Hence, any seeker of Truth can be seen as Hindu even if that seeker is Sikh, Christian, Muslim, or even atheist. Some would even go as far to say that it does not even matter if one seeks Truth, but as long as one approaches Truth through the actions of their life, they are truly Hindu.